9 Again, of the world he writes thus: “The belief commends itself to me that there was a world before this world and that after it there will be another. Do you wish to know that after the decay of this world there will be a new one? Hear the words of Isaiah: 'the new heavens and the new earth which I will make shall remain before me.' Do you wish to know that before the making of this world there have previously been others? Listen to the Preacher who says: 'the thing which has been, it is that which shall be; and that which is done is that which shall be done: and there is no new thing under the sun.
Is there anything whereof it may be said, See, this is new? It has been already of old time, which was before us.' A passage which proves not only that other worlds have been but that other worlds shall be; not, however, simultaneously and side by side but one after another.” And he immediately adds: “I hold that heaven is the abode of the deity, the true place of rest; and that it was there that reasonable creatures enjoyed their ancient bliss, before coming down to a lower plane and exchanging the invisible for the visible, they fell to the earth and came to need material bodies.
Now that they have fallen, God the creator has made for them bodies suitable to their surroundings; and has fashioned this visible world, and has sent into it ministers to ensure the salvation and correction of the fallen. Of these ministers some have held assigned positions and have been subject to the world's necessary laws; while others have intelligently performed duties laid upon them in times and seasons determined by God's plan. To the former class belong the sun, moon, and stars called by the apostle 'the creation.' and these have had allotted to them the heights of heaven.
Now the creation is subjected to vanity because it is encased in material bodies and visible to the eye. And yet it is 'made subject to vanity not willingly but by reason of him who has subjected the same in hope.' Others again of the second class, at particular places and times known to their Maker only, we believe to be His angels sent to steer the world.” A little farther on he says: “the affairs of the world are so ordered by Providence that while some angels fall from heaven others freely glide down to earth.
The former are hurled down against their will; the latter descend from choice alone. The former are forced to continue in a distasteful service for a fixed period; the latter spontaneously embrace the task of lending a hand to those who fall.” Again he writes: “whence it follows that these different movements result in the creation of different worlds; and that this world of ours will be succeeded by one quite unlike it. Now, as regards this falling and rising, this rewarding of virtue and punishment of vice, whether they take place in the past, present, or future, God, the creator, can alone apportion desert and make all things converge to one end.
For He only knows why He allows some to follow their own inclination and to descend from the higher planes to the lowest; and why He visits others and giving them His hand draws them back to their former state and places them once more in heaven.”
Source: Letters (New Advent)